Add a 'bot.dart' executable to the gardening tools.

tools/gardening/lib/src/util.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:args/args.dart';
 
@@ skipping 34 matching lines @@
 
 /// Reads the content of [uri] as text.
 Future<String> readUriAsText(HttpClient client, Uri uri) async {
   HttpClientRequest request = await client.getUrl(uri);
   HttpClientResponse response = await request.close();
   return await response.transform(UTF8.decoder).join();
 }
 
 ArgParser createArgParser() {
   ArgParser argParser = new ArgParser();
+  argParser.addFlag('help', help: "Help");
   argParser.addFlag('verbose',
       abbr: 'v', negatable: false, help: "Turn on logging output.");
   argParser.addOption('cache',
       help: "Use <dir> for caching test output.\n"
           "Defaults to 'temp/gardening-cache/'.");
   return argParser;
 }
 
 void processArgResults(ArgResults argResults) {
   if (argResults['verbose']) {
     LOG = true;
   }
   if (argResults['cache'] != null) {
     cache.base = Uri.base.resolve('${argResults['cache']}/');
   }
 }